Multiple Access Techniques for Wireless Communication FDMA TDMA SDMA PDMA A Presentation by...

Multiple Access Techniques for Wireless Communication

FDMA

TDMA

SDMA

PDMA

A Presentation by Schäffner Harald

Introduction

• many users at same time

• share a finite amount of radio spectrum

• high performance

• duplexing generally required

• frequency domain

• time domain

Frequency division duplexing (FDD)

• two bands of frequencies for every user

• forward band

• reverse band

• duplexer needed

• frequency seperation between forward band and reverse band is constant

frequency seperation

reverse channel forward channel

f

Time division duplexing (TDD)

• uses time for forward and reverse link

• multiple users share a single radio channel

• forward time slot

• reverse time slot

• no duplexer is required

time seperationt

forward channelreverse channel

Multiple Access Techniques

• Frequency division multiple access (FDMA)

• Time division multiple access (TDMA)

• Code division multiple access (CDMA)

• Space division multiple access (SDMA)

• grouped as:

• narrowband systems

• wideband systems

Narrowband systems

• large number of narrowband channels

• usually FDD

• Narrowband FDMA

• Narrowband TDMA

• FDMA/FDD

• FDMA/TDD

• TDMA/FDD

• TDMA/TDD

Logical separation FDMA/FDD

f

t

user 1

user n

forward channel

reverse channel

forward channel

reverse channel

...

Logical separation FDMA/TDD

f

t

user 1

user n

forward channel reverse channel


...

Logical separation TDMA/FDD

f

t

user 1 user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...

Logical separation TDMA/TDD

f

t

user 1 user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...

Wideband systems

• large number of transmitters on one channel

• TDMA techniques

• CDMA techniques

• FDD or TDD multiplexing techniques

• TDMA/FDD

• TDMA/TDD

• CDMA/FDD

• CDMA/TDD

Logical separation CDMA/FDD

code

f

user 1

user n



...

Logical separation CDMA/TDD

code

t

user 1

user n



...

Multiple Access Techniques in use

Multiple Access

Technique

Advanced Mobile Phone System (AMPS) FDMA/FDD

Global System for Mobile (GSM) TDMA/FDD

US Digital Cellular (USDC) TDMA/FDD

Digital European Cordless Telephone (DECT) FDMA/TDD

US Narrowband Spread Spectrum (IS-95) CDMA/FDD

Cellular System

Frequency division multiple access FDMA

• one phone circuit per channel

• idle time causes wasting of resources

• simultaneously and continuously transmitting

• usually implemented in narrowband systems

• for example: in AMPS is a FDMA bandwidth of 30 kHz implemented

FDMA compared to TDMA

• fewer bits for synchronization

• fewer bits for framing

• higher cell site system costs

• higher costs for duplexer used in base station and subscriber units

• FDMA requires RF filtering to minimize adjacent channel interference

Nonlinear Effects in FDMA

• many channels - same antenna

• for maximum power efficiency operate near saturation

• near saturation power amplifiers are nonlinear

• nonlinearities causes signal spreading

• intermodulation frequencies

Nonlinear Effects in FDMA

• IM are undesired harmonics

• interference with other channels in the FDMA system

• decreases user C/I - decreases performance

• interference outside the mobile radio band: adjacent-channel interference

• RF filters needed - higher costs

Number of channels in a FDMA system

• N … number of channels

• Bt … total spectrum allocation

• Bguard … guard band

• Bc … channel bandwidth

N=Bt - Bguard

Bc

Example: Advanced Mobile Phone System

• AMPS

• FDMA/FDD

• analog cellular system• 12.5 MHz per simplex band - Bt

• Bguard = 10 kHz ; Bc = 30 kHz

N=12.5E6 - 2*(10E3)

30E3= 416 channels

Time Division Multiple Access

• time slots

• one user per slot

• buffer and burst method

• noncontinuous transmission

• digital data

• digital modulation

Slot 1 Slot 2 Slot 3 … Slot N

Repeating Frame Structure

Preamble Information Message Trail Bits

One TDMA Frame

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.

Features of TDMA

• a single carrier frequency for several users

• transmission in bursts

• low battery consumption

• handoff process much simpler

• FDD : switch instead of duplexer

• very high transmission rate

• high synchronization overhead

• guard slots necessary

Number of channels in a TDMA system

• N … number of channels

• m … number of TDMA users per radio channel

• Btot … total spectrum allocation

• Bguard … Guard Band


N=m*(Btot - 2*Bguard)

Bc

Example: Global System for Mobile (GSM)

• TDMA/FDD

• forward link at Btot = 25 MHz

• radio channels of Bc = 200 kHz

• if m = 8 speech channels supported, and

• if no guard band is assumed :

N= 8*25E6200E3

= 1000 simultaneous users

Efficiency of TDMA

• percentage of transmitted data that contain information

• frame efficiency f

• usually end user efficiency < f ,

• because of source and channel coding

• How get f ?

Slot 1 Slot 2 Slot 3 … Slot N

Repeating Frame Structure

Preamble Information Message Trail Bits

One TDMA Frame

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.

Efficiency of TDMA

• bOH … number of overhead bits

• Nr … number of reference bursts per frame

• br … reference bits per reference burst

• Nt … number of traffic bursts per frame

• bp … overhead bits per preamble in each slot

• bg … equivalent bits in each guard time intervall

bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg

Efficiency of TDMA

bT = Tf * R

• bT … total number of bits per frame

• Tf … frame duration

• R … channel bit rate

Efficiency of TDMA

f … frame efficiency

• bOH … number of overhead bits per frame

• bT … total number of bits per frame

f = (1-bOH/bT)*100%

Space Division Multiple Access

• Controls radiated energy for each user in space

• using spot beam antennas

• base station tracks user when moving

• cover areas with same frequency:

• TDMA or CDMA systems

• cover areas with same frequency:

• FDMA systems

Space Division Multiple Access

• primitive applications are “Sectorized antennas”

• in future adaptive antennas simultaneously steer energy in the direction of many users at once

Reverse link problems

• general problem

• different propagation path from user to base

• dynamic control of transmitting power from each user to the base station required

• limits by battery consumption of subscriber units

• possible solution is a filter for each user

Solution by SDMA systems

• adaptive antennas promise to mitigate reverse link problems

• limiting case of infinitesimal beamwidth

• limiting case of infinitely fast track ability

• thereby unique channel that is free from interference

• all user communicate at same time using the same channel

Disadvantage of SDMA

• perfect adaptive antenna system: infinitely large antenna needed

• compromise needed

SDMA and PDMA in satellites

• INTELSAT IVA

• SDMA dual-beam receive antenna

• simultaneously access from two different regions of the earth


• COMSTAR 1

• PDMA

• separate antennas

• simultaneously access from same region


• INTELSAT V

• PDMA and SDMA

• two hemispheric coverages by SDMA

• two smaller beam zones by PDMA

• orthogonal polarization

Capacity of Cellular Systems

• channel capacity: maximum number of users in a fixed frequency band

• radio capacity : value for spectrum efficiency

• reverse channel interference

• forward channel interference

• How determine the radio capacity?

Co-Channel Reuse Ratio Q

• Q … co-channel reuse ratio

• D … distance between two co-channel cells

• R … cell radius

Q=D/R

Forward channel interference

• cluster size of 4

• D0 … distance serving station to user

• DK … distance co-channel base station to user

Carrier-to-interference ratio C/I

• M closest co-channels cells cause first order interference

C

=I

D0-n0

M

k=1DK

-nk

• n0 … path loss exponent in the desired cell

• nk … path loss exponent to the interfering base station

Carrier-to-interference ratio C/I

• Assumption:

• just the 6 closest stations interfere

• all these stations have the same distance D• all have similar path loss exponents to n0

C

I=

D0-n

6*D-n

Worst Case Performance

• maximum interference at D0 = R

• (C/I)min for acceptable signal quality

• following equation must hold:

1/6 * (R/D) (C/I)min=>-n

Co-Channel reuse ratio Q

• D … distance of the 6 closest interfering base stations

• R … cell radius

• (C/I)min … minimum carrier-to-interference ratio

• n … path loss exponent

Q = D/R = (6*(C/I)min)1/n

Radio Capacity m

• Bt … total allocated spectrum for the system


• N … number of cells in a complete frequency reuse cluster

m =Bt

Bc * Nradio channels/cell

Radio Capacity m

• N is related to the co-channel factor Q by:

Q = (3*N)1/2

m=Bt

Bc * (Q²/3)=

Bt

Bc *6 C

I3n/2( *( )min

)2/n

Radio Capacity m for n = 4

• m … number of radio channels per cell

• (C/I)min lower in digital systems compared to analog systems

• lower (C/I)min imply more capacity

• exact values in real world conditions measured

m =Bt

Bc * 2/3 * (C/I)min

Compare different Systems

• each digital wireless standard has different (C/I)min

• to compare them an equivalent (C/I) needed

• keep total spectrum allocation Bt and number of rario channels per cell m constant to get (C/I)eq :

Compare different Systems

• Bc … bandwidth of a particular system

• (C/I)min … tolerable value for the same system

• Bc’ … channel bandwidth for a different system

• (C/I)eq … minimum C/I value for the different system

CI

= CI

Bc

Bc’( ) ( )

min)²

eq * (

C/I in digital cellular systems

• Rb … channel bit rate

• Eb … energy per bit

• Rc … rate of the channel code

• Ec … energy per code symbol

C Eb*Rb Ec*Rc

I I I= =


• combine last two equations:

(C/I) (Ec*Rc)/I Bc’

(C/I)eq (Ec’*Rc’)/I’ Bc= = ( )²

• The sign ‘ marks compared system parameters


• Relationship between Rc and Bc is always linear (Rc/Rc’ = Bc/Bc’ )

• assume that level I is the same for two different systems ( I’ = I ) :

Ec Bc’Ec‘ Bc

= ( )³

Compare C/I between FDMA and TDMA

• Assume that multichannel FDMA system occupies same spectrum as a TDMA system

• FDMA : C = Eb * Rb ; I = I0 * Bc

• TDMA : C’ = Eb * Rb’ ; I’ = I0 * Bc’

• Eb … Energy per bit

• I0 … interference power per Hertz

• Rb … channel bit rate


Example

• A FDMA system has 3 channels , each with a bandwidth of 10kHz and a transmission rate of 10 kbps.

• A TDMA system has 3 time slots, a channel bandwidth of 30kHz and a transmission rate of 30 kbps.

• What’s the received carrier-to-interference ratio for a user ?

Example

• In TDMA system C’/I’ be measured in 333.3 ms per second - one time slot

C’ = Eb*Rb’ = 1/3*(Eb*10E4 bits) = 3*Rb*Eb=3*CI’ = I0*Bc’ = I0*30kHz = 3*I

• In this example FDMA and TDMA have the same radio capacity (C/I leads to m)

Example

• Peak power of TDMA is 10logk higher then in FDMA ( k … time slots)

• in practice TDMA have a 3-6 times better capacity

Capacity of SDMA systems

• one beam each user

• base station tracks each user as it moves

• adaptive antennas most powerful form

• beam pattern G() has maximum gain in the direction of desired user

• beam is formed by N-element adaptive array antenna


• G() steered in the horizontal -plane through 360°

• G() has no variation in the elevation plane to account which are near to and far from the base station

• following picture shows a 60 degree beamwidth with a 6 dB sideslope level


• reverse link received signal power, from desired mobiles, is Pr;0

• interfering users i = 1,…,k-1 have received power Pr;I

• average total interference power I seen by a single desired user:

Capacity of SDMA

i … direction of the i-th user in the horizontal plane

• E … expectation operator

I = E { G(i) Pr;I}K-1

i=1


• in case of perfect power control (received power from each user is the same) :

Pr;I = Pc

• Average interference power seen by user 0:

I = Pc E { G(i) }K-1

i=1


• users independently and identically distributed throughout the cell:

I = Pc *(k -1) * 1/D

• D … directivity of the antenna - given by max(G()) • D typ. 3dB …10dB

Pb = Q ( )


• Average bit error rate Pb for user 0:

3 D NK-1

• D … directivity of the antenna• Q(x) … standard Q-function• N … spreading factor• K … number of users in a cell

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